Difference between DDR1 & DDR2 RAM  

DDR2 is the most common type of memory in modern PCs today, having introduced features and functions that go well beyond the DDR SDRAM specification. It has 240 pins and is able to operate at data rates much faster than DDR1. Additionally, DDR2 adds a second port to transfer data packets to the CPU, doubling the transfer rate of DDR1.

DDR1 Originally known as plain 'DDR', this type of memory has become less common over the years, having been virtually replaced by DDR2. It's an evolution of SDRAM with 184 pins. While SDRAM transfers data once per clock signal, DDR transfers data twice, almost doubling the data transfer rate. DDR is Double data rate (DDR) SDRAM